Fitting in can be a tough business for kids, especially for the boy who wants to play dress-up and the girl with the short haircut. Now a new study finds that children who display such gender non-conformity — activity choices, interests and pretend play that don’t conform with what’s expected of their gender — are more likely to suffer physical, psychological and sexual abuse and experience post-traumatic stress disorder (PTSD) by early adulthood.

The abuse is most often perpetrated by parents or other adults in the household, the study found. And as many as 1 in 10 kids display gender non-conformity before age 11. It’s not clear why gender non-conforming kids suffer more abuse, but it may have to do with parents’ discomfort with their behavior. As lead author Andrea Roberts, a research associate in the department of society, human development and health at Harvard School of Public Health, toldUSA Today, parents “may have the idea, ‘If I force him not to be that way, he won’t be like that as an adult.'”

Interestingly, however, Roberts also noted that children are likely to display a wide variety of behaviors that have no connection to their future sexual orientation: 85% of gender-non-conforming children in the study were heterosexual in adulthood.

Yet boys who displayed gender non-conformity before age 11 were nearly three times as likely to suffer sexual abuse in childhood, compared with gender-typical boys. Non-conforming girls were 60% more likely to be abused sexually than conforming girls. Rates of physical and psychological abuse among non-conforming kids were similar across genders.

Harvard researchers gathered the data by administering childhood behavior questionnaires among nearly 9,000 young adults ages 17 to 27, who were enrolled in 1996 in the long-term Growing Up Today study. The questionnaires, given in 2007, asked participants to recall their childhood experiences: their favorite games and toys, media characters they had imitated or admired, and whether they took male or female roles in pretend play. The participants were also asked about physical, sexual and emotional abuse in childhood, and were screened for PTSD.

The researchers found that by young adulthood, rates of PTSD were almost twice as high among those who had been gender non-conforming as kids than among those who had not. PTSD has been linked to risky behaviors such as engaging in unprotected sex and to physical symptoms like cardiovascular problems and chronic pain, according to the study, published in Pediatrics.

“Parents need to be aware that discrimination against gender nonconformity…affects kids at a very young age, and has lasting impacts on health,” Roberts said in a statement.

While many children display temporary gender non-conformity during play, according to another study in the same issue of Pediatrics, others display much strong cross-gender behaviors. For them it goes beyond gender non-conformity — these children believe they were born in the wrong body. The study finds that a small but growing number of such teens and young children — many of whom may be diagnosed with gender identity disorder (GID) — are receiving counseling and medical sex-change treatment. By some estimates, as many as 1 in 10,000 children may have GID.

The study authors recommend early evaluation of children exhibiting GID, but hormonal sex-change treatment isn’t advised until puberty. Doctors note that treating kids and teens — despite the fact that long-term sex-hormone therapies can carry serious side effects like blood clots and cancer — is less damaging than doing nothing. Kids with GID may be more prone to stress, depression and suicide, and some may resort to self-mutilation to change their sex.

For the welfare of any child, Roberts says, parents must be aware that intolerance and abuse have lasting harmful effects. Roberts’ advice to parents, she told CBS, is: “accept your children and let go of how you thought things were going to be.”